1. Field of the Invention
The present invention relates to spectrophotometry. In particular, the present invention relates to in situ spectrophotometric measurements of fluids.
2. Discussion of Background
It is often necessary to make in situ measurements of fluids in wells, boreholes, storage and process tanks, and so forth. Applications include monitoring groundwater flow, studying the migration of subsurface contaminants, evaluating the progress of clean-up operations at toxic waste sites, detection of toxic or explosive substances, and monitoring the contents of industrial storage tanks.
Many different devices are available for making such measurements, responding to the chemical, electrical, or optical effects of some substance in the fluid. Optical devices with colorimetric and fluorescent indicators can measure parameters such as pH, oxygen or carbon dioxide concentration, the presence and concentration of metal ions, and the presence of organic contaminants such as PCBs. Some optical devices depend on an indicator which exhibits a change in an optical property, such as fluorescence emission, color, and so forth, in response to the fluid. The indicator may be in the form of a coating on an optical fiber. Light is directed through the fluid into the fiber, and changes in light absorption that result from the changed optical properties of the coating are detected.
A substance can often be identified simply by the frequency distribution of the light it absorbs, since the amount of light absorbed at different frequencies depends on the concentration of each component. A typical system for obtaining absorption spectra includes a light source, a sample cell which contains the fluid of interest, and a detector. Light from the source passes through the sample cell, then is focused onto the detector which measures the absorption spectrum of the light. Absorption spectra can be measured for solids, gases, or substances in solution. Measurements taken from a reference sample, such as distilled water, are compared to measurements taken from the fluid sample to help determine the concentrations of various components in the fluid sample.
Where measurements must be made at many different locations, a number of individual devices, or probes, may be connected to a single remotely-located measuring instrument. Measurements are then made by switching between each probe in turn. For monitoring purposes, the probes may be left in place for extended periods of time.
It is well known that retained air in a probe used for optical measurements of a fluid can lead to inaccurate measurements. Thus, irregular reflections from bubbles in the sample cell can affect the light transmission or scattering properties of the fluid. Arrangements for reducing colorimetry or turbidity measurement errors due to air or gas bubbles typically depend on vents and baffles to release bubbles separated from the measurement area of the fluid sample. For example, see Boe, et al. (U.S. Pat. No. 3,560,099), Muller (U.S. Pat. No. 3,819,278), Hach (U.S. Pat. No. 3,849,002), Leighton, et al. (U.S. Pat. No. 4,740,709).
In many applications, there is a need for measurements not only at a number of different locations, but at a range of subsurface depths at each location. For example, when monitoring groundwater transport at a given location, it is desirable to measure fluid flow at several depths. At present, this can only be accomplished by positioning a probe at each desired depth, at each location. The probes are often left in situ for long-term monitoring, which necessitates placing a number of probes at each location. A 100' (about 30.5 m) borehole may be only 3" (about 0.08 m) in diameter. A probe used therein must not only be compact, but must communicate reliably with a remote measuring instrument. Accurate positioning of a probe within such a long, narrow cavity is difficult; accurate deployment of many probes within the same cavity is even more difficult. There is no known apparatus which allows the efficient and accurate placement of a plurality of probes at predetermined positions in a single location.